Cited from an article on Khan Academy:
Jacob distributed a survey to his fellow students asking them how many hours they'd spent playing sports in the past day.
Does "the past day" mean yesterday or any arbitrary day in the past? Or does it have other meaning? Thank you in advance.
"The past day" can mean either yesterday or the previous 24 hours. The survey that went out to students was probably something like: How many hours did you spend playing sports yesterday? But the article is talking about the survey after it happened, so if the article used "yesterday" you would think it meant your (the reader's) yesterday. Instead they use "the past day" to indicate it was the survey-taker's yesterday, at the time they completed the survey.
As others have pointed out, there is ambiguity; the term might refer to the 24 hours immediately preceding the time when the survey-taker is answering the question, or it might refer to the most recent complete midnight-to-midnight 24-hour period. From the context of the article (a secondary source) it is not clear which is meant. Personally, if I were designing the survey I would use "yesterday."
The term "the past day" does not refer to any arbitrary day in the past, except in the sense that the day the survey was handed out may have been arbitrary. "A previous day" carries that sense of arbitrariness, which means it would not be good to use in a statistical survey (which is supposed to be a snapshot in time, and all survey-takers should be answering for the same time period).
It could either be the equivalent of "yesterday" - or it could mean "the past 24 hours". The meaning could extend into the past no further than that.
This usage is vague. While I will agree that the most likely intended meaning is "the past 24 hours", I would never use this where communication was critical, or where miscommunication was likely.
As an idiom, the past {timeperiod} is practically always used to mean "the period of time that began {timeperiod} ago and ending now." So, for me writing this right now at 2021-04-08T05:27:00Z:
For day in particular, "the past day" can also mean "yesterday", somewhat informally.
As @randomhead noted in his answer, the reason this language was used rather than just saying "yesterday" is because the time that the survey was published is not the same as the time the survey is filled out.
There is no exact meaning. It's ambiguous.
Arguably it means the previous day. Meaning, the day that ended on the midnight immediately before the question was asked. Although, in that case, the better phrase to use would have been "the previous day".
Arguably it means the 24-hour period that ended when the question was asked.
I would lean towards the latter.
The main thing to understand here is that "the past X" means "the most recent X". This is different from "a past X" which would mean "an X at some point before now". So "the past day" always refers to "the most recent day".
It's hard to be more precise than that, because the word "day" is often used informally and ambiguously; it could mean:
Sometimes, even the seemingly precise "24 hours" is used informally to mean "both day-time and night-time", to distinguish from "during the day" which means "the hours of daylight".
In the particular context given, it doesn't actually make much difference which of these definitions we pick. The survey is actually asking "how much of your time do you spend playing sport?", and asking about a particular day is just a way of getting the answer as a number.
